The emergence of animal life in oxygen-depleted conditions: insights from U isotopes – DYSOXIA

Variations in d238U measured in ancient sedimentary rocks are due to isotope fractionations (the distribution of U isotopes) occurring during redox processes and during sediment diagenesis. These fractionations are still poorly quantified and the calibration of U isotopes as a redox tracer requires their experimental determination.
This work involves the synthesis of minerals that are mainly iron-rich and partially reduced (e.g., green rust, magnetite, mackinawite, greenalite), and their reaction with dissolved uranium in an anoxic glovebox. Solid and dissolved products are collected throughout the experiment where U reduction occurs along with mineralogical transformations. Isotope signatures are then measured by MC-ICP-MS in order to determine U isotope fractionations occurring at each step, and mineralogical analyses are performed by TEM and XRD.

Our preliminary results indicate that abiotic reduction of U in anoxic and sulfide-rich environments is accompanied by a large isotope fractionation ~2‰. Additional experiments are being analysed.

The project includes the PI (Romain Guilbaud), a post-doctoral researcher (Pauline Méjean) working on the development and analysis of d238U in ancient sedimentary rocks, and a MSc student (Loren Tessier) working on the experimental determination of U isotope fractionations in a range of oxygen-depleted conditions.

Guilbaud R., Boulard E., Baptiste B., Delbes L., Menjot L. and Guarnelli Y. (2023) Burial of Fe bearing precursors to ‘chemical sediments’ in the deep past. In Goldschmidt 2023 Conference. GOLDSCHMIDT.

Complex life (e.g. animals) demands oxygen, and its emergence requires the transition from hostile, oxygen-free (anoxic) environments, to the well-oxygenated conditions we experience today. While our tools permit us to reconstruct the extent of extreme (anoxic or O2-rich) conditions, we currently lack the proxies to quantify intermediate (dysoxic) conditions, which may have characterized the environments where complex life emerged and evolved. Recently, we showed that modern-like oxygen minimum zones were established during the rise of animal life in late Precambrian. This project will focus on the calibration and the application of U isotopes to identify dysoxic conditions in these systems, in order to address how oxygen availability links to the evolution of early animals. The project comes at a key point in the current debate on animal evolution and ocean oxygenation, and it will constitute a fundamental step forward in reconstructing palaeo-environments throughout Earth’s history.